I really wanted to see the propeller in M13. I shot these with a 6” refractor on a full frame camera, so I started with a close crop that kept IC 4617 and the larger NGC 6207 in the frame. After aggressive triaging, I had a combination of about 150×120 images at gain zero and about the same number at gain 101. I weighted them by FWMH and stacked in Siril. These were OSC images so I used a very mild touch on the processing.
I found the key to bringing the propeller out was bringing up the local contrast in the core while holding a strong ceiling on the overall brightness. I used a series of masked GHS stretches to get this balance. I wish it was sharper, but I only applied BlurX at about 10% because higher levels began to accentuate the noise within the bright core of the cluster – leading to the classic orange peel noise structures. You might have to squint to see the propeller 🙂 but I didn’t want to push it further. I look forward to any comments!
by _g2v